Wire grid polarizers can be fastened inside of a cube. A cube polarizer can be better than a plate polarizer to (1) reduce astigmatism; (2) provide a mechanical structure, which can allow attachment of other devices (e.g. other polarizers or an LCOS imager); and (3) reduce wavefront distortion.
As shown in FIG. 13, a cube polarizer 130 can include a wire grid polarizer 131 sandwiched between two prisms—prismA 135 and prismB 136. The wire grid polarizer 131 can include wires 131w disposed over a substrate 131s. The cube polarizer 130 is not drawn to scale. In one example of a cube polarizer, the cube can be 10 millimeters (mm) wide, the substrate 131s can be 0.7 mm thick, and the wires 131w can be about 0.0003 mm thick. Thus, in order to show all components of the cube polarizer 130, the drawings have not been drawn to scale.
An unpolarized light beam U can enter one side (outer faceA) of prismA 135 and can be polarized into a reflected beam R and a transmitted beam T. The reflected beam R can reflect off the wires 131w of the wire grid polarizer 131, continue through prismA 135, and exit through another side (outer sideA) of prismA 135. The transmitted beam T can transmit through the polarizer 131 and prismB 136, and exit through a side (outer faceB) of prismB 136.
The reflected beam R has an optical path length OPLR and the transmitted beam T has an optical path length OPLT. The optical path length OPL is defined as the actual physical distance the light travels through the cube polarizer times an index of refraction n of the material(s) through which the light travels.
In some cube polarizer designs, there is a substantial difference in optical path length between the reflected and transmitted beams due to a thickness t of the substrate 131s (see FIGS. 13 and 14). For example, both prisms 135 and 136 can have the same size, and can be combined such that edges 137 of the prisms 135 and 136 align with edges of the wire grid polarizer 131. The wire grid polarizer 131 can be disposed at a 45° angle between the prisms 135 and 136, such that light entering perpendicularly to the outer faceA will meet the wire grid polarizer 131 at a 45° angle. The wire grid polarizer 131 can have wires 131w on one face of the wire grid polarizer 131. This cube may be physically symmetric based on outer dimensions, but not optically symmetrical due to the effect of the thickness t of the substrate 131s. Following are calculations showing this lack of optical symmetry. See reference variables in FIGS. 13 and 14 and definitions below. Note that FIG. 14 shows only the substrate 131s of the wire grid polarizer 131 without the wires 131w.
                    ⁢                            1.          ⁢                                          ⁢                      d            4            2                          =                              t            2                    +                      t            2                              ,                        d          4                =                                            2                        *                          t              .                                                          ⁢                                                          ⁢              2.                        ⁢                                                  ⁢                          OPL              R                                =                                                                      d                  1                                *                                  n                  p                                            +                                                d                  3                                *                                  n                  p                                            -                                                                                                                  d                        4                                            *                                              n                        p                                                              2                                    .                                                                          ⁢                  3.                                ⁢                                                                  ⁢                                  OPL                  R                                                      =                                                            d                  1                                *                                  n                  p                                            +                                                d                  2                                *                                  n                  p                                            -                                                t                  *                                      n                    p                                                                    2                                                                        ,              (                              d            2                    =                                                    d                3                            ⁢                                                          ⁢              and              ⁢                                                          ⁢                              d                4                                      =                                          2                            *              t                                      )                                ⁢                            4.          ⁢                                          ⁢                      OPL            T                          =                                                            d                1                            *                              n                p                                      +                                          d                2                            *                              n                p                                      -                                          2                            *              t              *                              n                p                                      +                                          2                            *              t              *                                                n                  s                                .                                                                  ⁢                5.                            ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              OPL                                =                                                                                    OPL                  T                                -                                  OPL                  R                                                                    =                                                                                -                                          2                                                        *                  t                  *                                      n                    p                                                  +                                                      2                                    *                  t                  *                                      n                    s                                                  +                                                      t                    *                                          n                      p                                                                            2                                                              =                                                                                                                  t                        *                                                  (                                                                                    2                              *                                                              n                                s                                                                                      -                                                          n                              p                                                                                )                                                                                            2                                                              .                                                                                  ⁢                                                                                  ⁢                    6.                                    ⁢                                                                          ⁢                  If                  ⁢                                                                          ⁢                                      n                    s                                                  =                                  n                  p                                                                        ,                        then          ⁢                                          ⁢          Δ          ⁢                                          ⁢          OPL                =                                            t              *                              n                p                                                    2                                .                    Reference variable definitions:                d1 is a distance from the outer faceA to a center of the polarizer 131.        d2 is a distance from the outer faceB to a center of the polarizer 131.        d3 is a distance from where the light is polarized to the outer sideA. Due to structural symmetry of the cube, d2 can equal d3.        d4 is a distance of travel of the transmitted beam 104 through the polarizer 131.        np is an index of refraction of the prisms (assuming both prisms have the same index).        ns is an index of refraction of the substrate. Any thin films on the substrate 131s are ignored as they are negligible relative to a thickness of the substrate 131s.        t is a thickness of the substrate. t is also a third leg of a triangle formed by d4 and t for a light beam U meeting the polarizer at a 45° angle.        ΔOPL is an absolute difference in optical path length between the transmitted beam T and the reflected beam R.        
This difference in optical path length
      Δ    ⁢                  ⁢    OPL    =            t      *              n        p                    2      can cause problems in some applications. Methods have been proposed to solve such problems, some of which may be impractical due to high manufacturing cost.
Curvature of a wire grid polarizer 131 in a cube can cause problems. The wire grid polarizer can curve due to stresses induced by the wires or other thin films adjacent to the wires. This curvature can result in a reflected light beam reflected off of one region of the polarizer having a different optical path length than a reflected light beam reflected off of another region of the polarizer, thus causing wavefront distortion. There can be a similar problem with the transmitted beam.
Information relevant to wire grid polarizers and polarizing cubes can be found in U.S. Pat. No. 8,467,128; U.S. Pat. No. 7,570,424; U.S. Pat. No. 7,085,050; U.S. Pat. No. 6,288,840; U.S. Patent Publication Number 2007/0297052; and in the publication “A new type of beam splitting polarizer cube,” Meadowlark Optics, Thomas Baur, 2005, pages 1-9.